Three separate 25.0 mL samples, each containing a different monoprotic weak acid of unknown identity (labeled Acid X, Acid Y, and Acid Z), are titrated individually with 0.150 M NaOH at 25 °C. The instructor provides the following key numerical features extracted from each titration curve:
Acid X Acid Y Acid Z
Half-equiv. pH 4.20 5.05 3.55
Vequiv (mL) 18.0 25.0 12.0
pH at equiv. 8.45 8.95 8.10
Use the information in the table to compare the three acids.
(a) Rank the acids X, Y, and Z in order of increasing acid strength, and justify your ranking quantitatively.
(b) For each acid, calculate the numerical value of Ka.
(c) Determine which acid sample originally contained the greatest number of moles of acid, and support your answer with calculations.
(d) Suppose a universal indicator that changes color sharply at pH 8.3 is used in place of a pH sensor. For which of the three titrations would the indicator yield the smallest titration error? Explain.